The present invention relates to a protective device for electrically-powered apparatus and equipment to prevent accidental electrocution of a user. Such a protective device is especially suitable for portable equipment powered by a power line through a power cord connection.
There have been many attempts in the prior art to produce a device which will prevent accidental electrocution. These devices work in a number of ways. Some operate by quickly detecting and reacting to a resultant overload. Others react to a short between a supply conductor and an external ground. Another type of device detects a short occurring in the load where a person may be connected between the shorted load and ground. Finally, there have also been many attempts to provide overload and/or over-voltage protection for circuits.
There are two types of prior art devices of particular interest in relation to the present invention. First, there is the Immersion Detection Circuit Interrupter ("IDCI"). Second, there is the Ground Fault Circuit Interrupter ("GFCI"). Both of these types of devices can be physically located in either the power outlet, in the plug, or in the apparatus itself.
IDCI's require a moisture sensor incorporated into the apparatus itself. The moisture sensor is typically in the form of metallic wires or conductors which will contact any water or other electrically conductive fluid which is introduced into the apparatus. The apparatus is typically used in damp areas such as the bathroom. The apparatus includes such things as immersion pumps, air bubble massage equipment, kitchen equipment, hair dryers, forced air heaters, etc.
The voltage-carrying parts and sensors in the IDCI's must be carefully installed and mechanically secured to avoid movement during normal operation and during shock loading conditions. In order to comply with the applicable safety regulations (UL-1664), prior to bringing a new product to market, a major design and engineering effort is required. Further, the design of such equipment is complicated by the fact that a sensor must be placed at or near every opening in the equipment.
The process for obtaining UL certification for equipment incorporating IDCI's is time consuming and expensive. Each particular equipment design must be tested by immersion in water and then, subsequently, the dried equipment is tested.
Another complication with the IDCI devices such as those disclosed in U.S. Pat. No. 4,589,047 is that the IDCI itself must be liquid-tight in order to prevent leakage of electrical power out of the submersed equipment. If the IDCI is not liquid tight, electricity would leak into the surrounding fluid when the appliance is accidentally submersed. Such liquid-tight devices are technically complicated to design and costly to produce.
German Patent DP 3702970 discloses an IDCI device located in the equipment plug. This type of device requires a three-wire cable. One of the wires is a shield wire, preferably of the wire-mesh type, wrapped around the other two wires and connected to the equipment sensors. UL-approved, flexible, shielded cables of this type are not currently produced in large quantities and thus the cost is prohibitively high.
UL-1664 requires that an IDCI device act to shut off the power supply within 3 AC half cycles or 25 milliseconds. However, the IDCI disclosed in U.S. Pat. No. 4,589,047 operates only during half of the AC current cycle. Thus, only one active half cycle or 8.33 milliseconds would be available to melt the fuse wire of the protective device. In order to achieve melting of the fuse wire in such a short time interval, a wire of 0.04 mm is required. Such a small fuse wire is extremely difficult to handle during manufacture. In addition, it has a very low tensile strength of the order of 120 grams and thus is susceptible to rupture resulting from mechanical shocks.
IDCI devices are also susceptible to trigger circuit malfunctions caused by interference from the power supply system. It is difficult and expensive to protect against this problem, further adding to the difficulties in engineering and manufacturing these devices.
GFCI devices contain either a mechanically-locked switch which can be magnetically unlocked or a magnetically-locked switch that can be unlocked by switching off the current to a magnetic spool. In either type of device, unlocking the switch results in shutting off the power supply.
U.S. Pat. No. 4,567,456 discloses a GFCI-type device wherein the switch is magnetically locked. This device suffers from the disadvantage that it requires a relatively high standby current consumption in order to keep the contacts normally closed. In addition, this device cannot operate at different voltages such as 120 V/220 V required for travel appliances. The magnetic locking spool required for 120 V operation will produce excessive heat when used at 220 V.
A major disadvantage of GFCI devices is the large number of electromagnetic and mechanical parts contained therein. Thus, GFCI devices are very expensive for installation in low-cost apparatus such as household equipment. In addition, the reliability of GFCI devices is limited by the number of moveable parts required and the periodic reset function tests.
GFCI type devices may be resettable so that after the GFCI trips, the user may reset and reuse the device.